The present invention relates to a print head assembly of a printing device, such as a typewriter, printer, or the like, in which armatures are swung by energizing coils, whereby print wires connected to the armatures are moved to a printing position for printing operation.
In a specific configuration of the print head assembly of this type, the armatures are pulled toward cores by means of the magnetic force of a permanent magnet, thereby accumulating an urging force in a resilient member, and holding the print wires in their rest position. When the coils are energized, a magnetic path formed by the permanent magnet is canceled so that the print wires are moved to the printing position by the urging force of the resilient member.
In order to increase the density of arrangement within a limited space, in the case of the aforementioned conventional configuration, a plurality of cores, each wound with a solenoid coil, are arranged in a circular ring, and a ring-shaped yoke member is disposed around the cores. Further, a ring-shaped permanent magnet is attached to the yoke member, thus forming a substantially columnar head.
Examples of such a prior art print head are disclosed in U.S. Pat. Nos. 4,225,250; 4,348,120; 4,411,538; and 4,618,277.
In these conventional arrangements using the print head of a circular general configuration, however, if the head is reduced in size, then the diameter of the permanent magnet is reduced naturally. Accordingly, the permanent magnet has a narrower area for each armature, so that a necessary magnetic force sometimes cannot be secured to hold the armatures in their rest position, against the urging force of the resilient member.
In general, in the prior art print head assembly constructed in this manner, various members are joined together, along an axis, between a head body and a guide member. The head body has a yoke, while the guide member has a nose for guiding the print wires. The joined members include the permanent magnet, a supporting member for swingably supporting the armatures, a spacer, a spring member opposed to the permanent magnet, etc. In order to align these members along the assembling axis, for example, the head body may be formed with positioning pins which are adapted to be fitted in holes in the supporting member or the spacer. Alternatively, the members may be joined together by separate fixing means, such as bolts penetrating them along the assembling axis.
In the print head assembly of this type, the position of each armature, relative to the core of its corresponding electromagnetic device, as well as the stroke of the armatures, must be determined accurately. It is therefore necessary to finish, by grinding or the like, the end face of each core and the contact surfaces of the head body, flush therewith, and the spacer on the head body.
Thus, the positioning pins, which project from the finished surfaces, cannot previously be formed integrally on the head body. In other words, separate positioning pins must be formed on the head body after the finishing work, which will complicate the manufacture.
Besides the aligning work, moreover, an assembling work using bolts or the like must be performed separately.
Conventionally, in assembling the print head assembly of the aforementioned type, the armatures are previously mounted on a resilient member, formed of a spring material, for example, thereby forming an armature unit. After the armature unit is attached to the nose of the guide member, the print wires are inserted individually into holes in the respective tip ends of the armatures, and into a guide portion of the nose. Then, the armatures and the print wires are fixed together by brazing or laser welding.
According to such a conventional method of assembling, however, the print wires and the armatures are fixed inside the head body. Therefore, a space for the fixing work must be kept between the tip ends of each two adjacent armatures.
As a result, the assembling work is troublesome and time-consuming, thus entailing an increased manufacturing cost.